Light absorption and filtering properties of vertically oriented semiconductor nano wires

ABSTRACT

A nanowire array is described herein. The nanowire array comprises a substrate and a plurality of nanowires extending essentially vertically from the substrate; wherein: each of the nanowires has uniform chemical along its entire length; a refractive index of the nanowires is at least two times of a refractive index of a cladding of the nanowires. This nanowire array is useful as a photodetector, a submicron color filter, a static color display or a dynamic color display.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/910,664, filed Oct. 22, 2010, which claims the benefit of U.S.Provisional Application No. 61/357,429, filed Jun. 22, 2010. Thisapplication is related to the disclosures of U.S. patent applicationSer. No. 12/204,686, filed Sep. 4, 2008 (now U.S. Pat. No. 7,646,943,issued Jan. 12, 2010), Ser. No. 12/648,942, filed Dec. 29, 2009 (nowU.S. Pat. No. 8,229,255, issued Jul. 24, 2012), Ser. No. 13/556,041,filed Jul. 23, 2012, Ser. No. 12/270,233, filed Nov. 13, 2008 (now U.S.Pat. No. 8,274,039, issued Sep. 25, 2012), Ser. No. 13/925,429, filedJun. 24, 2013, Ser. No. 13/570,027, filed Aug. 8, 2012 (now U.S. Pat.No. 8,471,190, issued Jun. 25, 2013), Ser. No. 12/472,264, filed May 26,2009 (now U.S. Pat. No. 8,269,985, issued Sep. 18, 2012), Ser. No.13/621,607, filed Sep. 17, 2012 (now U.S. Pat. No. 8,514,411, issuedAug. 20, 2013), Ser. No. 13/971,523, filed Aug. 20, 2013 (now U.S. Pat.No. 8,810,808, issued Aug. 19, 2014), Ser. No. 12/472,271, filed May 26,2009 (now abandoned), Ser. No. 12/478,598, filed Jun. 4, 2009 (now U.S.Pat. No. 8,546,742, issued Oct. 1, 2013), Ser. No. 14/021,672, filedSep. 9, 2013, Ser. No. 12/573,582, filed Oct. 5, 2009 (now U.S. Pat. No.8,791,470, issued Jul. 29, 2014), Ser. No. 14/274,448, filed May 9,2014, Ser. No. 12/575,221, filed Oct. 7, 2009 (now U.S. Pat. No.8,384,007, issued Feb. 26, 2013), Ser. No. 12/633,323, filed Dec. 8,2009 (now U.S. Pat. No. 8,735,797, issued May 27, 2014), Ser. No.14/068,864, filed Oct. 31, 2013, Ser. No. 14/281,108, filed May 19,2014, Ser. No. 13/494,661, filed Jun. 12, 2012 (now U.S. Pat. No.8,754,359, issued Jun. 17, 2014), Ser. No. 12/633,318, filed Dec. 8,2009 (now U.S. Pat. No. 8,519,379, issued Aug. 27, 2013), Ser. No.13/975,553, filed Aug. 26, 2013 (now U.S. Pat. No. 8,710,488, issuedApr. 29, 2014), Ser. No. 12/633,313, filed Dec. 8, 2009, Ser. No.12/633,305, filed Dec. 8, 2009 (now U.S. Pat. No. 8,299,472, issued Oct.30, 2012), Ser. No. 13/543,556, filed Jul. 6, 2012 (now U.S. Pat. No.8,766,272, issued Jul. 1, 2014), Ser. No. 14/293,164, filed Jun. 2,2014, Ser. No. 12/621,497, filed Nov. 19, 2009 (now abandoned), Ser. No.12/633,297, filed Dec. 8, 2009 (now U.S. Pat. No. 8,889,455, issued Nov.18, 2014), Ser. No. 12/982,269, filed Dec. 30, 2010, Ser. No.12/966,573, filed Dec. 13, 2010 (now U.S. Pat. No. 8,866,065, issuedOct. 21, 2014), Ser. No. 12/967,880, filed Dec. 14, 2010 (now U.S. Pat.No. 8,748,799, issued Jun. 10, 2014), Ser. No. 14/291,888, filed May 30,2014 Ser. No. 12/966,514, filed Dec. 13, 2010, Ser. No. 12/974,499,filed Dec. 21, 2010 (now U.S. Pat. No. 8,507,840, issued Aug. 13, 2013),Ser. No. 12/966,535, filed Dec. 13, 2010 (now U.S. Pat. No. 8,890,271,issued Nov. 18, 2014), Ser. No. 12/910,664, filed Oct. 22, 2010, Ser.No. 12/945,492, filed Nov. 12, 2010, Ser. No. 13/047,392, filed Mar. 14,2011 (now U.S. Pat. No. 8,835,831, issued Sep. 16, 2014), Ser. No.14/450,812, filed Aug. 4, 2014, Ser. No. 13/048,635, filed Mar. 15, 2011(now U.S. Pat. No. 8,835,905, issued Sep. 16, 2014), Ser. No.13/106,851, filed May 12, 2011, Ser. No. 13/288,131, filed Nov. 3, 2011,Ser. No. 14/334,848, filed Jul. 18, 2014, Ser. No. 14/032,166, filedSep. 19, 2013, Ser. No. 13/543,307, filed Jul. 6, 2012, Ser. No.13/963,847, filed Aug. 9, 2013, Ser. No. 13/693,207, filed Dec. 4, 2012,61/869,727, filed Aug. 25, 2013, Ser. No. 14/322,503, filed Jul. 2,2014, and Ser. No. 14/311,954, filed Jun. 23, 2014, Ser. No. 14/459,398,filed Aug. 14, 2014, Ser. No. 14/487,375, filed Sep. 16, 2014, Ser. No.14/501,983 filed Sep. 30, 2014, Ser. No. 14/503,598, filed Oct. 1, 2014,Ser. No. 14/516,162, filed Oct. 16, 2014, Ser. No. 14/563,781, filedDec. 8, 2014 are each hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

Nanostructures often exhibit fascinating physical properties not presentin their bulk counterparts. Optical properties of nanostructures havebeen one of the recent research focuses. Tuning optical properties ofnanostructures would facilitate their applications in the semiconductor,optics, and consumer electronics industry. In one example, opticalproperties of nanostructures can be controlled by their chemicalcomposition. Chemical doping can change electronic structures of thematerials semiconductor nanostructures are composed of, which in turnchanges their interaction with light. In another example, arrangingnanostructures into a regular lattice can yield optical propertiesindividual nanostructures lack. However, these conventional approachesoften require complex chemical synthesis or post-synthesis manipulation,and thus are less robust against minute variations of conditions andcannot easily and accurately position nanostructures in a functionaldevice. In contrast, the approach described herein overcomes theseproblems of the conventional approaches by harnessing small physicalsizes of nanostructures and a top-down fabrication process (i.e., partof a piece of bulk material is removed until desired nanostructures areachieved).

BRIEF SUMMARY OF THE INVENTION

Described herein is a nanowire array, comprising a substrate and aplurality of nanowires extending essentially perpendicularly from thesubstrate; wherein: a refractive index of the nanowires is at least twotimes of a refractive index of a cladding of the nanowires. Preferably anumber density of the nanowires is at most about 1.8/μm².

The nanowire array can be fabricated using a method comprising: (a)coating the substrate with a resist layer; (b) generating a pattern ofdots in the resist layer using a lithography technique; (c) developingthe pattern in the resist layer; (d) depositing a mask layer; (e)lifting off the resist layer; (f) forming the nanowires by dry etchingthe substrate; (g) optionally removing the mask player; wherein shapesand sizes of the dots determine the cross-sectional shapes and sizes ofthe nanowires.

The nanowire array can be used as a photodetector, a submicron colorfilter, a static color display or a dynamic color display.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective schematic of a nanowire array.

FIG. 1B shows a cross sectional schematic of the nanowire array of FIG.1A.

FIGS. 2A-2D are SEM images of an exemplary nanowire array.

FIG. 3A shows measured reflectance spectra of nanowire arrays withnanowires of a series of different radii.

FIG. 3B shows simulated reflectance spectra of the nanowire arrays ofFIG. 3A.

FIG. 3C shows dip positions in measured and simulated reflectancespectra of nanowire arrays, as functions of the radii of the nanowiresthereon.

FIG. 4A-4C show a major transverse component of the H_(1,1) mode atdifferent wavelengths, near a nanowire in an nanowire array.

FIG. 4D shows a schematic illustration of possible pathways of whitelight normally incident on the nanowire array.

FIG. 5A shows simulated effective refractive indexes (n_(eff)) of theH_(1,1) modes, as a function of wavelength, of three nanowire arrayswith different nanowire radii.

FIG. 5B shows simulated absorption spectra of the nanowire arrays ofFIG. 5A.

FIG. 5C compares a simulated absorption spectrum of the substrate in ananowire array, a simulated absorption spectrum of the nanowires (of 45nm radius) in the nanowire array, and a simulated reflectance spectrumof the entire nanowire array.

FIG. 6 shows a schematic top view of four pixels of the dynamic colordisplay comprising a nanowire array according to an embodiment.

FIGS. 7A and 7B show schematics of two exemplary apparatuses formeasuring reflectance spectra of the nanowire array.

DETAILED DESCRIPTION OF THE INVENTION

Described herein is a nanowire array, according to an embodiment,comprising a substrate and a plurality of nanowires extendingessentially perpendicularly from the substrate; wherein a refractiveindex of the nanowires is at least two times of a refractive index of acladding of the nanowires. A number density of the nanowires preferablyis at most about 1.8/μm².

According to an embodiment, a nanowire array comprises a substrate and aplurality of nanowires extending essentially perpendicularly from thesubstrate; wherein the nanowire array is operable as a submicron colorfilter. A “submicron color filter” as used herein means that an opticalfilter that allows light of certain wavelengths to pass through andoptical elements in the filter are less than a micron at least in onedimension.

According an embodiment, a nanowire array comprises a substrate and aplurality of nanowires extending essentially perpendicularly from thesubstrate; wherein the nanowires do not substantially couple. The term“substantially couple” as used herein means the nanowires collectivelyinteract with incident light such that spectral properties (e.g.,reflectance spectrum) of the nanowire array are distinct from spectralproperties of individual nanowire in the nanowire array. The term “thenanowires do not substantially couple” as used herein means one nanowiredoes not affect the properties of a neighboring nanowire. For example,when the pitch of the nanowires is changed and there is no color changeof the light absorbed or reflected by the nanowire, then the nanowiresdo not substantially couple.

According an embodiment, a nanowire array comprises a substrate and aplurality of nanowires extending essentially perpendicularly from thesubstrate; the nanowire array does not appear black to naked eye. Theterm “naked eye” as used herein means human visual perception that isunaided by enhancing equipment. The term “the nanowire array does notappear black to naked eye” as used herein means that the reflectedvisible light from the nanowire is substantially zero, which couldhappen under certain conditions based on the nanowire length, radius andpitch, as well as the optical properties of the substrate.

According to an embodiment, a nanowire as used herein means a structurethat has a size constrained to at most 1000 nm in two dimensions andunconstrained in the other dimension. An array as used herein means asystematic arrangement of objects such as a grid. The term “nanowiresextending essentially perpendicularly from the substrate” as used hereinmeans that angles between the nanowires and the substrate are from 85°to 90°. Cladding as used herein means a substance surrounding thenanowires, which can be vacuum, air, water, etc. A refractive index ofthe nanowires as used herein means a ratio of the speed of light invacuum relative to that in the nanowires. A number density of thenanowires as used herein means that an average number of nanowires perunit area of the substrate.

According to an embodiment, each of the nanowires in the nanowire arrayhas an essentially uniform chemical composition from one end of thenanowire to an opposite end of the nanowire in a longitudinal directionof the nanowire.

According to an embodiment, chemical composition of the nanowires asused herein means the simplest whole number ratio of atoms of eachelement present in the nanowires. The term “essentially uniform chemicalcomposition” as used herein means that the ratio of atoms varies at most3%, preferably at most 1%. A longitudinal direction of the nanowire asused herein means a direction pointing from one end of the nanowirefarthest from the substrate to one end of the nanowire nearest to thesubstrate.

According to an embodiment, each of the nanowires in the nanowire arrayis single crystalline, multi-crystalline or amorphous. That the nanowireis single crystalline as used herein means that the crystal lattice ofthe entire nanowire is continuous and unbroken throughout the entirenanowire, with no grain boundaries therein. That the nanowire ismulti-crystalline as used herein means that the nanowire comprisesgrains of crystals separated by grain boundaries. That the nanowire isamorphous as used herein means that the nanowire has a disordered atomicstructure.

According to an embodiment, the nanowires in the nanowire array arecomposed of a semiconductor or an electrically insulating material. Aconductor can be a material with essentially zero band gap. Theelectrical conductivity of a conductor is generally above 10³ S/cm. Asemiconductor can be a material with a finite band gap up to about 3 eVand general has an electrical conductivity in the range of 10³ to 10⁻⁸S/cm. An electrically insulating material can be a material with a bandgap greater than about 3 eV and generally has an electrical conductivitybelow 10⁻⁸ S/cm.

According to an embodiment, the nanowires in the nanowire array,comprise one or more materials selected from the group consisting of Si,Ge, GaN, GaAs, SiO₂, and Si₃N₄.

According to an embodiment, radii of the nanowires in the nanowire arrayare from 10 to 1000 nm; lengths of the nanowires are from 0.01 to 10 μm.

According to an embodiment, the nanowires and the substrate in thenanowire array have substantially the same chemical composition. Theterm “same chemical composition” as used herein means that the substrateand the nanowires are identical materials. The term “substantially same”here means the chemical composition differs by no more than 3%,preferably by no more than 1%.

According to an embodiment, the nanowires and the substrate in thenanowire array are single crystalline and the lattices of the nanowiresand the lattice of the substrate are continuous at interfacestherebetween. Namely, there is no grain boundary at the interfacesbetween the nanowires and the substrate.

According to an embodiment, the nanowires in the nanowire array arearranged in a predetermined pattern such as a rectangular grid, a squaregrid, concentric circle, hexagonal grid.

According to an embodiment, a distance of a nanowire to a nearestneighbor of the nanowire along a direction parallel to the substrate(also known as “pitch” or “pitch distance”) is at least 800 nm,preferably at most 10000 nm.

According to an embodiment, a reflectance spectrum of the nanowire arrayhas a dip; the dip position shifts to shorter wavelength with decreasingradii of the nanowires; and the dip position is independent from adistance of a nanowire to a nearest neighbor of the nanowire along adirection parallel to the substrate. A reflectance spectrum as usedherein means a ratio of the intensity of reflected light at a certainwavelength to the intensity of incident light at the same wavelength, asa function of wavelength. A “dip” in a reflectance spectrum as usedherein means that a region in the reflectance spectrum wherein thereflectance is smaller than the reflectance in surrounding regions ofthe reflectance spectrum. The “dip position” as used herein means thewavelength in the dip at which the reflectance is a minimum.

According to an embodiment, a reflectance spectrum of the nanowire arrayis independent from incident angles of illumination.

According to an embodiment, an incident angle as used herein means theangle between a ray of light incident on the substrate and the lineperpendicular to the substrate at the point of incidence.

According to an embodiment, a method of fabricating the nanowire arraycomprises: (a) coating the substrate with a resist layer; (b) generatinga pattern of dots in the resist layer using a lithography technique; (c)developing the pattern in the resist layer; (d) depositing a mask layer;(e) lifting off the resist layer; (f) forming the nanowires by dryetching the substrate; (g) optionally removing the mask player; whereinshapes and sizes of the dots determine the cross-sectional shapes andsizes of the nanowires.

According to an embodiment, a resist layer as used herein means a thinlayer used to transfer a pattern to the substrate which the resist layeris deposited upon. A resist layer can be patterned via lithography toform a (sub)micrometer-scale, temporary mask that protects selectedareas of the underlying substrate during subsequent processing steps.The resist is generally proprietary mixtures of a polymer or itsprecursor and other small molecules (e.g. photoacid generators) thathave been specially formulated for a given lithography technology.Resists used during photolithography are called photoresists. Resistsused during e-beam lithography are called e-beam resists. “Dots” as usedherein means discrete regions. A lithography technique can bephotolithography, e-beam lithography, holographic lithography.Photolithography is a process used in microfabrication to selectivelyremove parts of a thin film or the bulk of a substrate. It uses light totransfer a geometric pattern from a photo mask to a light-sensitivechemical photo resist, or simply “resist,” on the substrate. A series ofchemical treatments then engraves the exposure pattern into the materialunderneath the photo resist. In complex integrated circuits, for examplea modern CMOS, a wafer will go through the photolithographic cycle up to50 times. E-beam lithography is the practice of scanning a beam ofelectrons in a patterned fashion across a surface covered with a film(called the resist), (“exposing” the resist) and of selectively removingeither exposed or non-exposed regions of the resist (“developing”). Thepurpose, as with photolithography, is to create very small structures inthe resist that can subsequently be transferred to the substratematerial, often by etching. It was developed for manufacturingintegrated circuits, and is also used for creating nanotechnologyartifacts. Holographic lithography (also known as Interferencelithography) is a technique for patterning regular arrays of finefeatures, without the use of complex optical systems or photomasks. Thebasic principle is the same as in interferometry or holography. Aninterference pattern between two or more coherent light waves is set upand recorded in a recording layer (photoresist). This interferencepattern consists of a periodic series of fringes representing intensityminima and maxima. Upon post-exposure photolithographic processing, aphotoresist pattern corresponding to the periodic intensity patternemerges. A mask layer as used herein means a layer that protects anunderlying portion of the substrate from being etched. “Dry etching” asused herein means an etching technique without using a liquid etchant.

According to an embodiment, a method using the nanowire array 1 as aphotodetector comprises: shining light on the nanowire array; measuringphotocurrent on the nanowires; measuring photocurrent on the substrate;comparing the photocurrent on the nanowires to the photocurrent on thesubstrate. A photodetector as used herein means a sensor of light.

According to an embodiment, a method using the nanowire array as astatic color display comprises: determining locations and radii of thenanowires from an image to be displayed; fabricating the nanowires withthe determined radii at the determined locations on the substrate;shining white light on the nanowire array.

According to an embodiment, a dynamic color display comprises thenanowire array, an array of independently addressable white lightsources on a side of the substrate opposite the nanowires, wherein eachwhite light source corresponds to and is aligned in the substrate planewith one of the nanowires. “Independently addressable white lightsources” as used herein mean that each source can be controlled,adjusted, turned on or off, independently from other sources. “Whitelight” as used herein means a combination of visible light of differentwavelengths in equal proportions.

According to an embodiment, the white light sources in the dynamic colordisplay are white LEDs. LEDs are also known as light-emitting diodes.There are two primary ways of producing whitelight using LEDs. One is touse individual LEDs that emit three primary colors—red, green, andblue—and then mix all the colors to form white light. The other is touse a phosphor material to convert monochromatic light from a blue or UVLED to broad-spectrum white light, much in the same way a fluorescentlight bulb works.

According to an embodiment, in the dynamic color display, a first groupof the nanowires have a first radius, a second group of the nanowireshave a second radius, and a third group of the nanowires have a thirdradius, wherein the first group of the nanowires only allow red light topass, the second group of the nanowires only allow green light to pass,and the third group of the nanowires only allow blue light to pass.

According to an embodiment, a submicron color filter comprising thenanowire array, wherein each nanowire is placed on a photodetector,wherein only incident light with wavelengths in a dip of a reflectancespectrum of each nanowire is allowed reach the photodetector below. Amethod using the submicron color filter comprises shining white light onthe nanowire array, detecting transmitted light below the nanowires.

According to an embodiment, a ratio of a radius of the nanowires to apitch of the nanowires is at most 0.5.

EXAMPLES

FIGS. 1A and 1B show schematics of a nanowire array 100, according to anembodiment. The nanowire array 100 comprises a substrate 110 and aplurality of nanowires 120 extending essentially vertically from thesubstrate 110 (e.g. angles between the nanowires 120 and the substrate110 are from 85° to 90°). Each nanowire 120 preferably has uniformchemical composition along its entire length. Each nanowire 120 issingle crystalline, multi-crystalline or amorphous. The nanowires 120preferably are made of a suitable semiconductor or an electricallyinsulating materials, examples of which include Si, Ge, GaN, GaAs, SiO2,Si₃N₄, etc. A ratio of the refractive index (i.e., refractive indexcontrast) of the nanowires 120 and the refractive index of a cladding130 (i.e., materials surround the nanowires 120) is preferably at least2, more preferably at least 3. Radii of the nanowires 120 preferably arefrom 10 to 1000 nm, more preferably from 20 to 80 nm, most preferablyfrom 45 to 75 nm. Lengths of the nanowires 120 are preferably from 0.01to 10 μm, more preferably 0.1 to 5 μm. The nanowires 120 and thesubstrate 110 preferably have substantially the same chemicalcomposition. Crystal lattices of the nanowires 120 and the substrate110, if both are single crystalline, are preferably continuous atinterfaces therebetween. The nanowires 120 can have the same ordifferent shape and size. The nanowires 120 can be arranged in anysuitable pattern, examples of which include a rectangular grid, a squaregrid, a hexagonal grid, concentric rings, etc. A distance between ananowire 120 of the nanowire array 100 to a nearest neighbor nanowire ofthe nanowire array 100 along a direction parallel to the substrate isalso known as “pitch” or “pitch distance”. A ratio of the radius of thenanowires 120 to the pitch should not be too high, i.e., preferably atmost 0.5, more preferably at most 0.1. If this ratio is too high, thenanowires 120 substantially couple to each other (i.e., the nanowires120 collectively interact with incident light such that spectralproperties (e.g., reflectance spectrum) of the nanowire array 100 aredistinct from spectral properties of individual nanowire 120 in thenanowire array 100) and the nanowire array 100 appears black to nakedeyes and cannot function as color filters or displays. Preferably, thenumber density of the nanowires 120 (average number of nanowires 120 perunit area on the substrate 110) is thus at most about 1.8/μm².Preferably, the pitch of the nanowires 120 is at least 500 nm.

FIGS. 2A-2D show exemplary scanning electron microscope (SEM) images ofthe nanowire array 100. In these exemplary SEM images, 10,000 nanowires120 consisting of silicon are arranged in a 100 μm×100 μm square grid onthe substrate 110 consisting of silicon, wherein a distance of onenanowire to a nearest neighbor nanowire of the nanowire array 100 alonga direction parallel to the substrate is about 1 μm. The length of thenanowires 120 are about 1 μm. The radius of the nanowires 120 is about45 nm.

FIG. 3A shows measured reflectance spectra of five nanowire arrays 100,each of which consists 10,000 nanowires 120 consisting of siliconarranged in a 100 μm×100 μm square grid on the substrate 110 consistingof silicon, wherein the pitch of these nanowire arrays 100 and thelength of the nanowires 120 are about 1 μm. These five nanowire arrays100 are identical except that the nanowires 120 thereof have uniformradii of 45 nm, 50 nm, 55 nm, 60 nm, 65 nm and 70 nm, respectively.Under white light illumination, these nanowire arrays 100 appear to bedifferent colors (e.g., red, green, blue, cyan, etc.) to naked eyes. Thereflectance spectrum of each of these nanowire arrays 100 shows one dip,i.e., incident light at wavelengths within the dip is reflected at alesser proportion compared to incident light at wavelengths outside thedip. Positions of the dip dictates the apparent colors of the nanowirearrays 100. For example, if the position of the dip is between 700 and635 nm, the nanowire array 100 appears cyan; if the position of the dipis between 560 and 490 nm, the nanowire array 100 appears magenta; ifthe position of the dip is between 490 and 450 nm, the nanowire 100appears yellow. Position of the dip progressively shifts to shorterwavelength from about 770 nm in the nanowire array 100 with the largestnanowires 120 (70 nm in radius) to about 550 nm in the nanowire array100 with the smallest nanowires 120 (45 nm in radius). The positions ofthe dips in these five nanowire arrays 100 range across the entirevisible spectrum. The position of the dip is independent from the pitchof the nanowire array 100, which indicates that the dips are not due todiffractive or coupling effects. Although diffractive and couplingeffects are not required, the nanowire array 100 can function when sucheffects are present. The nanowire array 100 preferably has a pitchgreater than 800 nm so that diffractive and coupling effects do notdominate. The magnitude of the dips decreases with increasing pitchesbecause greater pitch leads to lower number density of the nanowires120. FIG. 3A also illustrates that magnitudes of the dips increase withthe positions of the dips in wavelength, due to strong materialdispersion of the substrate material above its bandgap (i.e., therefractive index of the substrate 110 increases at wavelengths above thebandgap of the material thereof while the effective refractive index ofa guided mode in the nanowires 120 remains close to the refractive indexof air, which leads to higher refractive index contrast between theguided mode and the substrate 110 and thus stronger reflectance in thedip, i.e., smaller magnitude of the dip, at shorter wavelengths). For ananowire array with thicker nanowires, more than one dip may be presentin its reflectance spectrum and the nanowire array may appear in acombination of colors.

The reflectance spectra can be measured with focused or collimatedincident illumination. In an exemplary measurement as shown in FIG. 7A,incident white light from a light source 810 is focused by 20× objectivelens 830 (numerical aperture=0.5); reflected light is collected by thesame objective lens 830 and partially reflected by a beam splitter 820to a spectrometer 850. An iris 840 is used at the image plane of theobjective lens 830 to reject any light other than light reflected by thenanowire array 100. In another exemplary measurement as shown in FIG.7B, incident white light from a light source 815 is collimated by a lens835 and directed to the nanowire array 100 through a beam splitter 825;reflected light is collected by a 10× objective lens 865 to aspectrometer 855. An iris 845 is used at the image plane of theobjective lens 865 to reject any light other than light reflected by thenanowire array 100. A silver mirror can used to measure absoluteintensity of reflected light, which is used to calculate (i.e.,normalize) the reflectance spectra. The reflectance spectra are found tobe essentially independent from the incident angle, which indicates thatthe reflectance spectra are dominated by coupling dynamics betweennormal component of the incident light and the nanowire array 100.

FIG. 3B shows simulated reflectance spectra of the five nanowire arrays100 in FIG. 3A using the finite difference time domain (FDTD) method.The FDTD method is a method of numerically simulating propagation oflight in a structure and can be used to predict detailed characteristicsof the propagation. The simulated reflectance spectra are quantitativelyin good agreement with the measured reflectance spectra of FIG. 3A, withrespect to the dip position as a function of nanowire radius. Comparedto the measured reflectance spectra, simulated spectra have shallowerdips, which could be due to a reflectivity difference between roughenedsubstrate surface in actual nanowire arrays and ideally flat substratesurface presumed in the simulation. Lumerical's (Lumerical Solutions,Inc.) FDTD and MODE solvers were used to perform the simulation. Twodimensional models were constructed in MODE solver by simply specifyingnanowire radius, pitch and material properties. A periodic boundarycondition is then imposed in the substrate plane. These modes were usedto study the evolution of the fundamental mode of the nanowires 120 as afunction of wavelength. Full three dimensional models were constructedin Lumerical's FDTD solver by specifying complete nanowire geometryalong with pitch and material properties. Periodic boundary conditionsin the substrate plane and absorbing boundary conditions along the zaxis (normal direction of the substrate 110) were imposed. A plane wavepulse source of the appropriate bandwidth was launched along the z axisand monitors placed to compute the total absorbed, transmitted andreflected fluxes as a function of wavelength. The nanowires 120 and thesubstrate 110 were assumed to be silicon in the simulation.

FIG. 3C shows the positions of the dips as a function of radii of thenanowires 120 in both of the measured and simulated reflectance spectra,which shows an essentially linear dependence on the nanowire radii. Theessentially linear dependence indicates a strong correlation oragreement between the measured and simulated reflectance spectra.

Wavelength selective reflection of the nanowire array 100 as shown inFIGS. 3A and 3B originates from strong wavelength dependence of fielddistribution of the fundamental guided mode (HE_(1,1) mode) of eachnanowire 120. The fundamental guided mode as used herein means theguided mode with the lowest frequency. The guided mode of a nanowire 120as used herein means a mode whose field decays monotonically in thetransverse direction (directions parallel to the substrate 110)everywhere external to the nanowire 120 and which does not lose power toradiation. Symmetry prevents efficient interaction between the nanowire120 and other guided mode, and the nanowire 120 is too small to supporthigher order HE_(1,m) modes (guided modes with higher frequency). FIGS.4A-4C show a major transverse component (e.g. E_(y)) (a field componentperpendicular to the direction of propagation of the mode) of theH_(1,1) mode at different wavelengths. At wavelengths in the dip of thereflectance spectrum, the field distribution of the HE_(1,1) mode ofeach nanowire 120 is characterized by a transverse field that ispartially contained in the nanowire 120 and partially extends into thecladding 130, as shown in FIG. 4A. Incident light at these wavelengthscan efficiently excite the HE_(1,1) mode and be guided by the nanowire120 to the substrate 110 or be absorbed by the nanowire 120. The largerefractive index contrast between the nanowire 120 and the claddingleads to non-negligible longitudinal field component (E_(z)) (i.e.,field component parallel to the direction of propagation of the mode)which has significant overlap with the nanowire 120; since the modalabsorption is proportional to the spatial density of electromagneticenergy, which includes E_(z), incident light at these wavelengths canboth efficiently couple to (i.e., a significant portion of the incidentlight propagates inside the nanowire 120) and be absorbed by thenanowire 120. At wavelengths well below the dip of the reflectancespectrum, the field distribution of the HE_(1,1) mode of each nanowire120 is characterized by a transverse field essentially confined in thenanowire 120 due to large refractive index contrast between the nanowire120 and the cladding, as shown in FIG. 4B. Incident light at thesewavelengths cannot efficiently excite the HE_(1,1) mode and thus cannotbe efficiently guided or absorbed by the nanowire 120; incident light atthese wavelengths is substantially reflected by an interface of thesubstrate 110 and the cladding 130. At wavelengths well above the dip ofthe reflectance spectrum, the field distribution of the HE_(1,1) mode ofeach nanowire 120 is characterized by a transverse field essentiallyexpelled from the nanowire 120, as shown in FIG. 4C. Incident light atthese wavelengths can efficiently excite the HE_(1,1) mode but theHE_(1,1) mode at these wavelengths cannot be efficiently guided orabsorbed by the nanowire 120; incident light at these wavelengths issubstantially reflected by an interface of the substrate 110 and thecladding. FIG. 4D shows schematic illustration of possible pathways ofwhite light normally incident on the nanowire array 100. Light ofwavelengths beyond the dip in the reflectance spectrum is reflected bythe substrate 110; light of wavelengths in the dip is guided by thenanowire 120 to transmitted through the substrate 110 or absorbed by thenanowire 120.

The position of the dip of the reflectance spectrum is determined by theradius of the nanowire 120. FIG. 5A shows simulated effective refractiveindexes (n_(eff)) of the H_(1,1) modes, as a function of wavelength, ofthree nanowire arrays 100 with different nanowire radii (45 nm, 55 nmand 70 nm in traces 501, 502 and 503, respectively), wherein n_(eff) areobtained by the FDTD method over a 1 μm by 1 μm unit cell under periodicboundary conditions, the material of the nanowire arrays 100 is assumedto be silicon, the cladding is assumed to be air, and length of thenanowires 120 is assumed to be 1 μm. When light propagates in a mediumthat comprises materials of different indices of refraction, the lightbehaves as if it propagates in a uniform medium with a uniform index ofrefraction whose value is some intermediate of those of the materials.This uniform index is referred to as the effective refractive index. Aperiodic boundary condition is a set of boundary conditions that areoften used to model a large system as an infinite periodic tile of asmall unit cell.

In each trace, n_(eff) increases sharply and approaches n_(Si)(refractive index of silicon) for wavelengths shorter than thecorresponding dip position in FIG. 3A. The dip occurs where n_(eff)asymptotes to n_(air) (refractive index of air). n_(eff) as a functionof wavelength (also called a dispersion curve) shifts to longerwavelength with increasing nanowire radius.

FIG. 5B shows simulated absorption spectra (obtained by the FDTD method)of the nanowire arrays 100 of FIG. 5A (traces 511, 512 and 513corresponding to nanowire arrays with nanowires of 45 nm, 55 nm and 70nm radii, respectively). For blue light (<500 nm) over 90% of theH_(1,1) mode can be absorbed in a 1 μm length of the nanowire. FIG. 5Ccompares a simulated absorption spectrum 521 of the substrate 110 in thenanowire array 100 with nanowires 120 of 45 nm radius (corresponding totraces 501 and 511), a simulated absorption spectrum 523 of thenanowires 120 of 45 nm radius in this nanowire array 100, and asimulated reflectance spectrum 522 of this nanowire array 100. The dipin the reflectance spectrum 522 is slightly redshifted relative to thepeak in the absorption spectrum 523 of the nanowires 120, whichindicates that the long wavelength edge of the dip arises more fromcoupling to the substrate 110. Nonetheless, this shows that the guidedlight is in fact absorbed in the nanowires 120, and so the shape of thereflectance spectrum 523 and the amount of light absorbed in thenanowires 120 can be controlled by altering the length thereof. Thelight absorbed by the substrate 110 (see trace 521) can be enhanced ordiminished by the nanowires 120 relative to light absorption of a planarsubstrate, depending on whether the nanowires 120 absorb or merelycouple to the substrate 110. The fact that the filtering characteristicsof the nanowire array 100 are related to absorption in different partsthereof can lead to useful applications in optoelectronic devices.

A method of fabricating the nanowire array 100 includes (a) coating thesubstrate 110 with a resist layer (e.g. e-beam resist, photo resist,etc.); (b) generating a pattern of dots in the resist layer using alithography technique (e.g. photolithography, e-beam lithography,holographic lithography, etc.); (c) developing the pattern in the resistlayer; (d) depositing a mask layer (e.g. Al, Cr, SiO₂, Si₃N₄, Au, Ag,etc.); (e) lifting off the resist layer; (f) forming the nanowires 120by dry etching the substrate 110; (g) optionally removing the maskplayer; wherein shapes and sizes of the dots determine thecross-sectional shapes and sizes of the nanowires 120. The resist can bepoly(methyl methacrylate) (available from MicroChem located in Newton,Mass.). The mask layer can be aluminum deposited by a suitable techniquesuch as e-beam evaporation, thermal evaporation, sputtering, etc. Themask layer can be about 40 nm thick. The substrate 110 can be a singlecrystalline silicon wafer. Dry etching can be conducted in aninductively coupled plasma-reactive ion etcher (such as those availablefrom Surface Technology Systems, located at Redwood City, Calif.). Anexemplary dry etching process includes alternating etch and depositionsteps at room temperature, wherein 60 sccm of SF₆ and 160 sccm of C₄F₈gases were used therein, respectively. The mask layer can be removedusing a suitable etchant (e.g. Type A aluminum etchant available fromTransene Company Inc. located in Danvers, Mass.) or solvent (e.g. acid,base, or organic solvent). SEM images can be taken in an SEM such asZeiss Ultra55 available from Carl Zeiss NTS located at Peabody, Mass.

A method using the nanowire array 100 as a photodetector comprises:shining light on the nanowire array 100; measuring photocurrent on thenanowires 120; measuring photocurrent on the substrate 110; comparingthe photocurrent on the nanowires 120 to the photocurrent on thesubstrate 110.

The nanowire array 100 can also be used as a submicron color filter. Forexample, each of the nanowires 120 in the nanowire array 100 can beplaced on a photodetector. Only incident light with wavelengths in thedip of the reflectance spectrum of a nanowire can reach thephotodetector below this nanowire. A method using the nanowire array 100as a submicron color filter comprises shining white light on thenanowire array 100, detecting transmitted light below the nanowires 120.

A method using the nanowire array 100 as a static color displaycomprises: determining locations and radii of nanowires from an image tobe displayed; fabricating the nanowires with the determined radii at thedetermined locations on the substrate; shining white light on thenanowire array. The word “static” here means that the display can onlyshow one fixed image. By appropriate choice of individual nanowireplacement and radius in the nanowire array 100, the nanowire array 100can display a color image under white light illumination.

The nanowire array can also be used in a dynamic color display. The word“dynamic” here means that the display can display different images atdifferent times. The dynamic color display, according to one embodiment,comprises the nanowire array 100, an array of independently addressablewhite light sources on a side of the substrate 110 opposite thenanowires 120, wherein each white light source corresponds to and isaligned in the substrate plane with one of the nanowires 120. Thenanowires 120 can have predetermined radii and thus only allow light ofdesired wavelengths from the light sources to pass. For example, FIG. 6shows a schematic top view of four pixels of the dynamic color display.Nanowires 715, 725, 735 and 745 respectively correspond to and arealigned with white light sources 710, 720, 730 and 740. The white lightsources can be white LEDs. The nanowire 715 has a radius of about 45 nmand only allows red light to pass. The nanowires 725 and 735 have aradius of about 60 nm and only allows green light to pass. The nanowire745 has a radius of about 70 nm and only allows blue light to pass. Theindependently addressable white light sources can be replaced by ascanning white light beam.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A nanowire array, comprising a substrate and aplurality of nanowires extending essentially perpendicularly from thesubstrate, wherein a ratio of a radius of the nanowires to a pitch ofthe nanowires is at most 0.5, wherein radii of the nanowires are from 10to 1000 nm; lengths of the nanowires are from 0.01 to 10 μm.
 2. Ananowire array of claim 1; wherein: a refractive index of the nanowiresis at least two times of a refractive index of a cladding of thenanowires.
 3. A nanowire array of claim 1, wherein a number density ofthe nanowires is at most about 1.8/μm2.
 4. The nanowire array of claim1, wherein each of the nanowires is single crystalline,multi-crystalline or amorphous.
 5. The nanowire array of claim 1,wherein the nanowires are composed of a semiconductor.
 6. The nanowirearray of claim 1, wherein the nanowires comprise one or more materialsselected from the group consisting of Si, Ge, GaN, GaAs, SiO2, andSi3N4.
 7. The nanowire array of claim 1, wherein the nanowires and thesubstrate are single crystalline and the lattices of the nanowires andthe lattice of the substrate are continuous at interfaces therebetween.8. The nanowire array of claim 1, wherein the nanowires are arranged ina predetermined pattern.
 9. The nanowire array of claim 1, wherein adistance of a nanowire to a nearest neighbor of the nanowire along adirection parallel to the substrate is at least 800 nm.
 10. The nanowirearray of claim 1, wherein a reflectance spectrum thereof has a dip; thedip position shifts to shorter wavelength with decreasing radii of thenanowires; and the dip position is independent from a distance of ananowire to a nearest neighbor of the nanowire along a directionparallel to the substrate.
 11. The nanowire array of claim 1, wherein areflectance spectrum thereof is independent from incident angles ofillumination.
 12. A method of fabricating the nanowire array of claim 1,comprising: generating a pattern of dots in a resist layer using alithography technique; forming the nanowires by etching the substrate;wherein shapes and sizes of the dots determine the cross-sectionalshapes and sizes of the nanowires.
 13. The method of claim 12, furthercomprising: coating the substrate with the resist layer; developing thepattern in the resist layer; depositing a mask layer; lifting off theresist layer; and optionally removing the mask player.
 14. The method ofclaim 12, wherein the etching is dry etching.
 15. A method using thenanowire array of claim 1 as a photodetector comprises: shining light onthe nanowire array; measuring photocurrent on the nanowires; measuringphotocurrent on the substrate; comparing the photocurrent on thenanowires to the photocurrent on the substrate.
 16. A method using thenanowire array of claim 1 as a static color display comprises:determining locations and radii of the nanowires from an image to bedisplayed; fabricating the nanowires with the determined radii at thedetermined locations on the substrate; shining white light on thenanowire array.
 17. A color filter comprising the nanowire array ofclaim 1, wherein each nanowire is placed on a photodetector, whereinonly incident light with wavelengths in a dip of a reflectance spectrumof each nanowire is allowed reach the photodetector below.
 18. A methodusing the color filter of claim 17 comprises shining white light on thenanowire array, detecting transmitted light below the nanowires.
 19. Thenanowire array of claim 1, wherein the nanowires are composed of anelectrically insulating material.
 20. The nanowire array of claim 1,wherein the nanowire array is operable as a submicron color filter. 21.The nanowire array of claim 1, wherein at least one nanowire among theplurality of nanowires has a dip in a reflectance spectrum of the atleast one nanowire, wherein a light of a wavelength in the dip incidenton the at least one nanowire is guided by the at least one nanowire tobe transmitted through the substrate.
 22. The nanowire array of claim21, wherein the dip is at an IR wavelength.
 23. The nanowire array ofclaim 21, wherein the nanowire array is operable as an infrared lightfilter.
 24. The nanowire array of claim 21, wherein the at least onenanowire comprises GaAs.
 25. The nanowire array of claim 21, wherein theat least one nanowire has a diameter of between about 70 nm and about500 nm.
 26. A dynamic display comprises the nanowire array of claim 21.27. A photodetector comprising the nanowire array of claim
 21. 28. Thenanowire array of claim 21, wherein the nanowire array is operable as alight filter.
 29. The nanowire array of claim 21, wherein the nanowiresdo not substantially couple.
 30. A dynamic color display comprises ananowire array comprising a substrate and a plurality of nanowiresextending essentially perpendicularly from the substrate, the nanowirearray being operable as a color filter; an array of independentlyaddressable white light sources on a side of the substrate opposite thenanowires, wherein each white light source corresponds to and is alignedin the substrate plane with one of the nanowires.
 31. The dynamic colordisplay of claim 30, wherein the white light sources are white LEDs or ascanning white light beam.
 32. The dynamic color display of claim 30,wherein a first group of the nanowires have a first radius, a secondgroup of the nanowires have a second radius, and a third group of thenanowires have a third radius, wherein the first group of the nanowiresonly allow red light to pass, the second group of the nanowires onlyallow green light to pass, and the third group of the nanowires onlyallow blue light to pass.